A huge market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable transducer head for reading and writing data. The HSA forms part of a servo control system that positions the moveable transducer head over a particular track on the disk to read or write information from and to that track, respectively.
When manufacturing a disk drive, servo sectors may be written to a disk to define a plurality of radially-spaced concentric circumferential tracks. Each servo sector may include at least a track identification (TKID) field, a sector ID field having a sector ID number to identify the sector, and a group of servo bursts (e.g. an alternating pattern of magnetic transitions) which the servo control system of the disk drive samples to align the moveable transducer head with or relative to a particular track. Typically, the servo control system moves the transducer head toward a desired track during a “seek” mode using the TKID field as a control input. Once the moveable transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the moveable transducer head over that track in a “track follow” mode. During track follow mode, the moveable transducer head repeatedly reads the sector ID field of each successive servo sector to obtain the binary encoded sector ID number that identifies each sector of the track. In this way, the servo control system continuously knows where the moveable transducer head is relative to the disk.
An external servo writer may be used to write the servo sectors to the disk during disk drive manufacturing. Servo writers often employ extremely accurate head positioning mechanics, such as laser interferometers or optical encoders, to ensure that the servo sectors are written at the proper radial location and extremely accurate clocking systems may be utilized in order to write the servo sectors in the proper circumferential locations on the disk.
Alternatively, many disk drives have the capability to self-servo write servo sectors. During self-servo writing, the internal electronics of the disk drive are used to write the servo sectors. One technique used in self-servo writing disk drives is for the head of the disk drive to write a plurality of spiral reference patterns to the disk which are then processed by the disk drive to write the servo sectors along a circular path. For example, the spiral reference patterns may be written by moving the head from an outer diameter of the disk to an inner diameter of the disk. In the self-servo writing process, the head then writes the final servo sectors by servo-ing on the seeded spiral patterns.
However, during the spiral reference pattern writing process, errors may occur that affect the radial and circumferential position and slope of the spiral reference patterns and degrade the spiral patterns. These types of errors include vibration of the HDA, acceleration and de-acceleration errors, flutter, windage on the head and arm, flex circuit bias, temperature, etc. Errors in writing the spiral patterns may then propagate into the writing of the servo sectors, thereby degrading the operating performance of the disk drive and reducing the manufacturing yield.
Further, as disk drive manufacturers have been forced to increase data capacity in disk drives to remain competitive, a greater number of tracks per inch (TPI) are required to be servo-written to each disk to provide for increased data storage capacity. To accomplish this, the distance between each of the servo-written tracks has become increasingly smaller which often results in track squeeze errors (TSEs) wherein servo bursts deviate from their normal positions. TSEs may cause increased servo control errors during track following resulting in degraded performance, reliability issues, and even disk drive failure.
It is therefore desirable to employ techniques to accurately identify errors in the spiral reference patterns before self-servo writing occurs in order to maximize TPI and to minimize TSEs.